The mitogen-activated protein kinase kinases (the MAPK/ERK kinases; MKKs or MEKs) and their downstream substrates, the extracellular-regulated kinases have been intensively studied for their roles in development and disease. Until recently, it had been assumed any mutation affecting their function would have lethal consequences. However, the identification of MEK1 and MEK2 mutations in developmental syndromes as well as chemotherapy-resistant tumors, and the discovery of genomic variants in MEK1 and MEK2 have led to the realization the extent of genomic variation associated with MEKs is much greater than had been appreciated. In this review, we will discuss these recent advances, relating them to what is currently understood about the structure and function of MEKs, and describe how they change our understanding of the role of MEKs in development and disease.